Many hydrocarbon gases such as natural gas, coal mine and coal seam gases, landfill gases, refinery operation off-gases and hydroprocessing recycle loop gases contain one or more light components that either contaminate the main gas or that are themselves more valuable if they can be separated from the main gas stream. Such light gases include nitrogen and hydrogen. A number of economic considerations make it desirable to separate these light gases from a hydrocarbon gas stream.
Absorption using a physical solvent to remove the heavier hydrocarbon components and therefore separate them from the light components can be employed. This process is described in several patents, including U.S. Pat. Nos. 4,623,371, 4,680,042, 4,740,222, 4,832,718, 4,883,514, 5,224,350, 5,325,672, 5,462,583, 5,551,972, 6,698,237 B2, 7,337,631 B2, 7,442,847 B2, and 7,563,307 B2 along with U.S. patent application Ser. No. 12/082,976, all which are incorporated by reference herein in their entirety. These patents describe absorption/flash regeneration systems for removal of light components such as nitrogen or hydrogen from heavier components such as methane or ethylene. In most of the referenced prior art, the feed gas and the lean solvent stream are chilled to between 0° F. (−17.7° C.) and −40° F. (−40° C.) to enhance recovery of the heavier components and to reduce contamination of the light component stream with heavier components, including solvent components. The absorber may operate at a wide range of pressures, typically 200 psig (13.8 barg) or higher. The last flash used to release the recovered heavier components from the rich solvent is operated at low pressure to minimize the concentration of absorbed heavier components in the lean solvent.
In these processes the heavier components are absorbed away from the light components using a circulating physical solvent. Reducing the pressure of the rich solvent in one or more flash separators releases the heavier absorbed component and regenerates the solvent for recirculation to the absorber. These patents address systems wherein the physical solvent used is external, meaning a made up of component(s) added to the system (U.S. Pat. Nos. 4,623,371, 4,680,042, 4,740,222, 4,832,718, 4,883,514) and also systems wherein the physical solvent used is internally generated and is therefore composed of heaviest component(s) in the feed gas (U.S. Pat. Nos. 5,462,583 and 5,551,972). Controlling the amount of light components in the rich solvent affects the recovery of the light component and the purity of the absorbed and released heavier component. In some applications, the vapor released from the first flash vessel is recycled to the bottom of the absorber as stripping gas, effectively reducing the amount of light component in the heavier component product released from the rich solvent in later flash vessels (U.S. Pat. Nos. 4,740,222, 4,832,718, 5,462,583, 5,551,972). Using this method, lowering the pressure of the first flash vessel will result in a less light component in the heavy product. This first flash pressure must be lower when the amount of light component in the feed is higher in order to maintain a similar heavier component purity. The amount of light component absorbed and released with the heavier component product can alternatively be controlled by recycle of heavy component product to the bottom of the absorber as stripping gas (U.S. Pat. No. 5,325,672), or with a reboiler on the absorber bottoms. When heavy component recycle is used, more recycle results in a purer heavy component, and a higher rate of recycle is required to maintain heavy product purity when the feed gas contains more light component.
A specification for the concentration of light component in the heavy component product can be controlled over a wide range of feed gas compositions using either the heavy product recycle method or the first flash vapor recycle method. However, a design to accommodate a low amount of light component in the feed, and also perform with a high concentration results in design of each piece of equipment to handle the worst case load of the two operations required. An improvement to the process that results in improved flexibility to accommodate changes in feed composition and improved operability while minimizing or even reducing associated system cost and complexity is needed.